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Question DetailsAsked on 10/12/2017

Why is the drip pan under the water heater full of water, leaking after turning heat on for the first time/season?

Slow constant drip leaking into the garage directly below tank.

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1 Answer


Here are some previous similar questions about water in the catch pan FYI:

Bear in mind hot water heater water is probably 120-140 degrees so be careful. If a boiler, more like boiling (200 F range),

Since you just cranked it up (and I presume you mean this is a steam or hot water boiler for the heating system, not a domestic "water heater" - though answer would be pretty much the same in either case) - first thing I would be looking at is gaskets/seals on the circulating pump, and dielectric couplings (which look about like this) -

Though leaks at them are much more common when the system is turned off and the metal shrinks, than at startup - but changing the gskets for them is cheap and easy.

Another likely place would be the overpressure/overtemp relief valve which comes off near the top of the tank and should have a metal discharge pipe to within an inch or two of the pan - when system kicks back up these commonly leak a bit when they are older, especially if not exercised annually. Could try bailing out the pan and then releasing some water through the valve (usually you lift and then release a lever to test it, discharges about a cup ro two fo water with a short opening time) and see if it drips or stops dripping.

Could also be the drain valve at the bottom is leaking (common with plastic ones especially) - if so could replace, or just put a gasketed brass hose threaded cap like this on it with teflon plumbers tape to act as the watertight seal -

Otherwise, could be a leak in one of the water pipes (in the pipe or at the tank entry/exit point), around the thermostat probe into the tank (or heating element ports if electric), or potentially a leaking tank itself - which held water OK when cold but a crack or pinhole opened up when it heated up and expanded. Or even a pipe leak above it running down the outside of a pipe to the unit. So check all around for signs of wetness or leakage, working from top to bottom.

If a boiler, then sometimes the internal seals (as opposed to the mounting flange seals) on the circulating pump go bad when not used for a while, so if it is leaking right out of a pump that can commonly run another couple hundred to maybe $400-500 to fix depending on brand.

If dripping into the firebox from within the unit, provided pipes coming into/going out the top are not leaking at the pipe or threaded fitting into the tank (which you should be able to see from above), then that is bad news - pretty much certain to be a leaking tank, which cannot be fixed.

One thing you pretty much know, since the pan is full, is it is not just condensation from the flue - it is definitely a leak somewhere.

If you can't determine where the leak is after bailing out the pan, then Plumbing would be your Search the List category to get it looked at - though if much beyond its rated life you might just save that service call and bite the typically (for 50 gallon or less heater) $1000-1500 cost for a new one if this is a regular tank-type domestic water heater (except in CA where I read they now have to be high-efficiency so more in the $2000-3000 range). Of course, if a boiler or tankless heater you are talking several to a bunch of thousands of $ so some inspection cost (typically $100-150 plus commonly $50-100 more for the repair unless leaking pump) by a plumber to see if just a leaking pipe or fitting might well pay off.

Bear in mind of course, if a plumber looks at a heater leaking like that, he is going to be mighty tempted to sell you a new one - maybe even if the tank itself (about the only leak point not redfily fixable) is not the problem. Some will make the inspection visit free if they then end up putting in a new unit for you - though of course there is a lot of temptation to tellyou need a new unit so the more you can do to see for yourself (alone or with his pointing out the leak point) to make certain it is not repairable before jumping into a new heater, the better. Water heaters tend to last just about their rated life for short-life ones (under 6 years), ones with 6 year or longer life I have found generally last jusat about that is not maintained, or about 25-50% longer than rated life if some water is drained each year and sacrificial anodes are replaced as needed in bad/aggressive water areas.

Answered 3 years ago by LCD

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